Thanks for the patch guys.
I'm seeing a 5-10ish fps increase, and much smoother gameplay with very little/no stuttering at medium settings. True fullscreen seems to work fine too, though I have seen some more land/sea glitches (the land bars, grey lines on the hud target arrows, that kind of thing). Closing & restarting the game seems to solve this, at least temporarily.
As for Crossfire though... it works, but it's a huge performance drop over a single card for me. I get a good stable 30fps with no shadows and mostly medium settings on a single 5770, with crossfire it jumps wildly from 5-25fps every second or two, never settling down. Crossfire also still stutters heavily, but not as bad as it did before.
Definite step in the right direction, but Crossfire and indeed ATI cards in general need a lot more work. Not tried multiplayer so nothing to report there.